One way to propose

An impulsive, but also thrifty, pilot considers proposing by displaying the question on a sign pulled by a small airplane. Assume that the airplane will fly horizontally at a constant speed of 130 km/hr and that the sign is a very thin, nearly rigid, plane sheet with a length of 30 m and a height of 2 m.
If the specific consumption of the airplane fuel is 0.8×10-6 kg/J, the specific gravity of the fuel is 0.97, the price of the fuel is 0.95 $/litre and the efficiency of the engine is 0.73, calculate the cost for one hour of towing the sign, in addition to the cost of flying the airplane itself.

Contributed by Stavros Tavoularis, Department of Mechanical Engineering, University of Ottawa, Ottawa, Canada.

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